The present proposal aims to understand the relationship between hearing, vocal development, and vocal learning in a small Australian parrot - the budgerigar or domesticated parakeet. Budgerigars learn new vocalizations throughout life with a capacity that may be unlimited. For a number of reasons, budgerigars provide a better animal model for studying some of the biological processes underlying human speech and language learning than the more well-studied songbirds. The present proposal involves a blend of behavioral physiological, and anatomical studies on hearing, perception, and the production of vocal signals, and the learning of new vocalizations through operant conditioning. Bringing vocal learning under operant control in an animal model provides a whole new range of opportunities to explore the relation between auditory perception and vocal production. Past auditory discrimination studies with simple and complex sounds have now led to a model of the budgerigar auditory system functions and is specialized for perceiving vocal signals and new experiments, using a linear systems approach, are now focused on testing aspects of that model. Vocal learning brought under operant control will test the role of auditory feedback, temporary threshold elevation, short-term memory, and short- term memory, and other behavioral mechanisms known to be involved in human speech and language. Achieving control over vocal learning is a major breakthrough and now allows a rigorous functional test of the role of the role various nuclei and pathways in the auditory vocal circuit suspected to be involved in vocal learning and auditory memory. Additional experiments are aimed at understanding the role of steroids in sex differences in vocal perception and vocal learning. Together, these experiments seek to identify and understand the basic biological principles that are capable of organizing and maintaining a complex, learned vocal communication system that remains plastic throughout adulthood. Results from these experiments should have relevance understanding the perception of complex natural sounds such as vocalizations ans speech, the effects of hearing (and hearing loss) on vocal learning, the design of sensorimotor interfaces and neural circuits that sustain complex learning, and the evolution of acoustic communication systems including human language.